High frequency transmitting windows



April 21, 1959 R. s. B LACKADDER ETAL 2,883,631

HIGH FREQUENCY TRANSMITTING WINDOWS Filed July 19, 1955 FIG. 1.

FIG.2.

I12 UCR'ZZODG .ERL. fifila/akaddep QIB.M Vaughan P Yo my .power.transmitting high frequency power are having a window which is madeUnited States i atent 2,883,631 HIGH FREQUENCY TRANSMITTING WINDOWSRobert Lennox Scott Blackadder, Blaekheath, Guildford, James RodneyMitchell Vaughan, Stolte Pages, and Peter Young, West Ealing, London,England, assignors to Electric & Musical Industries Limited, Hayes,Mir?- dlesex, England, a company of Great Britain Application July 19,1955, Serial No. 522,988

Claims priority, application Great Britain July 27, 1954 12 Claims. (Cl.333-98) This invention relates to devices having windows fortransmitting high frequency electromagnetic power such as are requiredfor sealing an evacuated device whilst at the same time permit-ting thetransmission of high frefinding a satisfactory glass which can withstandthe heat which is generated during transmission of high degrees ofMoreover, if refractory materials capable of employed for such windowsthen it is also found that these windows give rise to reflections whichit is found not only causes the oscillations generated in the klystronor magnetron to change their mode of oscillation but also alfects theresonant frequency of the generated oscillations.

The object of the present invention is to provide an improved devicehaving a window which is capable of withstanding such high powers whilstat the same time reducing radio frequency reflections which might arise.

According to the present invention a device is provided for transmittinghigh frequency electromagnetic power of refractory material capable oftransmitting high frequency power and having a single optic axis, saidwindow being sealed to an apertured support, said optic axis being soorientated and the arrangement being such that radio frequencyreflections which arise in said window and at the edge of the aperturein said support are substantially mutually cancelling.

In one example of the present invention the window is preferably made ofsapphire and is sealed between a pair of apertured diaphragms and theoptic axis of said window is arranged to be parallel to the axis alongwhich power is transmitted through said window. In another example ofthe invention the window is sealed to a single diaphragm and inthis-case the optic axis to be oblique to the axisalong through thewindow.

In order that the said invention may be clearly understood and readilycarried into effect, it will now be described with reference to theaccompanying drawings, in which- Figure 1 illustrates in longitudinalsections a device in accordance with one embodiment of the invention,and

Figure 2 is a similar view of a device in accordance with anotherembodiment of the invention.

As shown in Figure 1 of the drawings, the reference numeral 1illustrates a section of waveguidewhich may be connected to the resonantcavity of an electron discharge valve such as a kly'stron or magnetronand is proof the window is arranged which power is transmitted videdwith a window indicated at 2 which as shown is positioned in a planenormal to the axis of the waveguide l and which serves to seal the endof the waveguide 1 in a vacuum tight manner. The window 2 is made ofcrystalline refractory material having a single optic axis and which iscapable of transmitting high frequency power. Optic axis is defined byJenkins and White in Funda mentals of Optics (McGraw-Hill PublishingCo.) as be ing the one and only one direction through a crystal in whichordinary and extraordinary raysbehave alike in all respects. In thepresent invention the optic axis of the window is so orientated inrelation to the axis of the waveguide 1, i.e. the axis along which highfrequency power is transmitted through the waveguide, that reflectionswhich arise in said window and at the edges of a support for said windoware substantially mutually cancelling. The window 2 is preferably madein the form of a disc of artificial sapphire (crystalline alumina), thedisc being so formed for the embodiment shown in Figure 1 that the opticaxis thereof is normal to the plane of said disc. The disc is mounted soas to close an aperture in a support, which in the embodiment shown inFigure 1 comprises a pair of apertured diaphragms 3 and 4 between whichthe disc is sealed, the rims 5 and 6 of the latter being .in turn sealedas by welding to the end of the waveguide 1. The diaphragms 3 and 4 aremade of a material having a coefficient of expansion lying in the range5.7 to 5.9x 10- such as a nickel-cobalt-iron alloy for example amaterial known by the registered trademarks Kovar or Nicosel. The twodiaphragms 3 and 4 and the window 2 are sealed together by applying apowdered glass, having a coefficicnt of expansion of about 5.7 x 10-such as a glass having a composition of about 67% SiO 21% B 0 4 /2% K 0,4% A1 0 and 3% Na o such as one known by the registered trademark Kodialaround the edge of the disc 2 as indicated at 7 and to the adjacentmetal surfaces of the diaphragms 3 and 4 and heating'the assembly toabout 900 C. to fuse the powdered glass. The diaphragm 3 is, as shown,spaced from the end of the waveguide 1 which is of the usual rectangularcrosssection. The body 8 in which the waveguide 1 is formed is ofcircular cross-section and the end wall is provided with the usualannular high frequency choke 9, whilst the opposite longer walls of thewaveguide 1 at their mid positions are provided with chamfered portions10 .of known form in order to reduce the danger of sparking between theend of the waveguide and the window 2. With such an arrangement withklystron or magnetron can be sealed in a vacuum tight manner and at thesame time high frequency power can be transmitted through the window 2into a further waveguide 11, the end of which is spaced from butdisposed adjacent to the window 2. The body 12 in which the Waveguide 11is formed is provided with a high frequency choke 13 and the longerwalls of the waveguide 11 are also provided with chamfered portions 14.The expansion of the sapphire disc in the plane of the disc is 5.7 1()per degree centigrade which matches the material known by the registeredtrademark Kovar sufficiently well for a sound hermetic seal to beproduced. The expansion along the optic axis 15 of the sapphire disc is6.4 10- which would cause a mismatch if it were in or near the plane ofthe disc. With such an arrangement it is found that reflections of highfrequency power not only occur in the window 2 but also at the edges ofthe diaphragms 3 and 4. These reflections can be reduced bysuitablyorientating the optic axis 15 of the window 2 and by a suitable choiceof the diameter of the apertures and the thickness of the window 2.These are the main factors which determine the reduction of reflection!but other factors also enter into consideration, namely, the gapbetween the diaphragm 3 and the end of the waveguide 1 and the chamferedportions 10. The

optimum disposition of the various parts and their dimensions can bestbe determined by trial and error but in one example which is suitablefor use with power of a frequency of 35,000 megacycles per second anarrangement having the following dimensions is found to give goodresults: The cross-sectional dimensions of the waveguide 1 are 0.280inch 0.l40 inch; the thickness of the window 2 is 0.0055 inch with adielectric constant of 9.4; the optic axis of the window is parallel tothe axis of the waveguide 1; the thickness of diaphragms 3 and 4 is0.010 inch, the diameter of the apertures in the diaphragms are 0.205inch; the gap between the diaphragm 3 and the end of the waveguide 1 is0.008 inch; the diameter of the choke 9 is 0.345 inch and the depth ofthe choke is 0.086 inch, and the radius of the chamfered portions 10 is0.078 inch. The waveguide 11 which is coupled to the waveguide 1 may bespaced from the diaphragm 4 by a distance of 0.016 inch.

The dielectric loss of artificial sapphire is found to be rather lessthan that of the best low-loss glass at millimetre wavelengths.Moreover, the dielectric constant is more than twice as high (9.4against 4.3) so that, the thickness for a matched window is less thanhalf that of a glass window. The cooling of the window which mainlyarises by convection from the outer face of the window is, therefore, atleast twice as effective and this, coupled with the high melting pointof sapphire which is over 2,000 C., affords a window with a powerhandling capacity which is more than twice that of a glass window.

In the embodiment of the invention shown in Figure 2 of the drawingswhere similar reference numerals have been given to similar parts,instead of sealing the window 2 between a pair of diaphragms as shown inFigure l, the window 2 is only sealed to a single diaphragm 3 and inorder to obtain cancellation of reflections occurring from said windowand from the edge of the diaphragm 3 the optic axis, instead of beingdisposed parallel to the axis of the waveguide 1, as in the arrangementshown in Figure 1 is disposed obliquely to the axis as indicated by thedotted line 16. When the optic axis is not perpendicular to the plane ofthe sapphire disc then both ordinary and extraordinary rays arepropagated in the disc and give rise to reflections having anappreciable difference of phase. These reflections combine to give anapparent equivalent reflecting plane which does not coincide with thecentral plane of the disc. Thus, by suitable adjustment of the opticaxis this apparent reflecting plane can be made to coincide with theplane of the diaphragm so that a radio frequency match can be obtainedwith only one diaphragm. Preferably this one diaphragm is disposed onthe vacuum side of the window so that the limiting peak power at whichthe window assembly will spark is considerably raised. The obliquity ofthe optic axis will not be found to be so great as appreciably todisturb the thermal expansion match to the diaphragm.

The dimensions of the parts for producing a substantial reduction inreflection with the arrangement shown in Figure 2 for a frequency of35,000M/c.s. are as follows:

The cross-sectional dimensions of the waveguide 1 are 0.280 inch 0.140inch; the thickness of the window 2 is .006 inch with a dielectricconstant of 9.4; the optic axis of the window makes an angle of 40 withthe axis of the waveguide 1 and lies in that plane through the waveguideaxis which makes an angle of 20 with the narrow side of the waveguide;the thickness of the diaphragm 3 is 0.010 inch; the diameter of theaperture in the diaphragm is 0.205 inch; the gap between the diaphragm 3and the end of the waveguide 1 is .008 inch; the diameter of the choke 9is 0.345 inch and the depth of the choke is 0.086 inch and the radius ofthe chamfered portions 10 is 0.078 inch. The waveguide 11 which iscoupled to the waveguide 1 may be spaced from the diaphragm 3 by adistance of 0.022 inch.

What we claim is:

1. A device for transmitting high frequency power comprising awaveguide, a crystalline window disposed in a plane normal to the axisof the waveguide and made of a refractory material having a single opticaxis and capable of transmitting high frequency power, an aperturedsupport to which said window is sealed, said optic axis being soorientated to effect substantially mutual cancellation of radiofrequency reflections which arise in said window and at the edge of theaperture in said support.

2. A device for transmitting high frequency power comprising awaveguide, a crystalline plane window disposed in a plane normal to theaxis of the waveguide and made of a refractory material having a singleoptic axis and capable of transmitting high frequency power, anapertured support to which said window is sealed, said optic axis beingso orientated to eifect substantially mutual cancellation of radiofrequency reflections which arise in said window and at the edge of theaperture in said support.

3. A device for transmitting high frequency power comprising awaveguide, a window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support to which saidwindow is sealed, the optic axis of the window being so orientated toeffect substantially mutual cancellation of radio frequency reflectionswhich arise in said window and at the edge of the aperture in saidsupport.

4. A device for transmitting high frequency power comprising awaveguide, a plane window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support to which saidwindow is sealed, the optic axis of the window being so orientated toelfect substantially mutual cancellation of radio frequency reflectionswhich arise in said window and at the edge of the aperture in saidsupport.

5. A device for transmitting high frequency power comprising awaveguide, a crystalline window disposed in a plane normal to the axisof the waveguide and made of a refractory material having a single opticaxis and capable of transmitting high frequency power, an aperturedsupport comprising two apertured metal diaphragms between which saidwindow is sealed, said optic axis being orientated parallel to the axisalong which said high frequency power is transmitted to effectsubstantially mutual cancellation of radio frequency reflections whicharise in said window and at the edges of the apertures in saiddiaphragms.

6. A device for transmitting high frequency power comprising awaveguide, a crystalline plane window disposed in a plane normal to theaxis of the waveguide and made of a refractory material having a singleoptic axis and capable of transmitting high frequency power, anapertured support comprising two apertured metal diaphragms betweenwhich said window is sealed, said optic axis being orientated parallelto the axis along which said high frequency power is transmitted toeffect substantially mutual cancellation of radio frequency reflectionswhich arise in said window and at the edges of the apertures in saiddiaphragms.

7. A device for transmitting high frequency power comprising awaveguide, a window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support comprising twoapertured metal diaphragms between which said window is sealed, theoptic axis of the window being orientated parallel to the axis alongwhich said high frequency power is transmitted to effect substantiallymutual cancellation of radio frequency reflections which arise in saidwindow and at the edges of the apertures in said diaphragms.

8. A device for transmitting high frequency power comprising awaveguide, a plane window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support comprising twoapertured metal diaphragms between which said window is sealed, theoptic axis of the window being orientated parallel to the axis. alongwhich said high frequency power is transmitted to effect substantiallymutual cancellation of radio frequency reflections which arise in saidwindow and at the edges of the apertures in said diaphragms.

9. A device for transmitting high frequency power comprising awaveguide, a crystalline window disposed in a plane normal to the axisof the waveguide and made of a refractory material having a single opticaxis and capable of transmitting high frequency power, an aperturedsupport comprising a single apertured metal diaphragm to which saidwindow is sealed, said optic axis being orientated obliquely to the axisalong which said high fre quency power is transmitted to effectsubstantially mutual cancellation of radio frequency reflections whicharise in said window and at the edge of the aperture in said diaphragm.

10. A device for transmitting high frequency power comprising awaveguide, a crystalline plane window disposed in a plane normal to theaxis of the waveguide and made of a refractory material having a singleoptic axis and capable of transmitting high frequency power, anapertured support comprising a single apertured metal dia phragm towhich said window is sealed, said optic axis being orientated obliquelyto the axis along which said high frequency power is transmitted toeffect substantially mutual cancellation of radio frequency reflectionswhich arise in said window and at the edge of the aperture in saiddiaphragm.

11. A device for transmitting high frequency power 6 comprising awaveguide, a window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support comprising a singleapertured metal diaphragm to which said window is sealed, the optic axisof the window being orientated obliquely to the axis along which saidhigh frequency power is transmitted to effect substantially mutualcancellation of radio frequency reflections which arise in said windowand at the edge of the aperture in said diaphragm.

12. A device for transmitting high frequency power comprising awaveguide, a plane window disposed in a plane normal to the axis of thewaveguide and made of sapphire, an apertured support comprising a singleapertured metal diaphragm to which said window is sealed, the optic axisof the window being orientated obliquely to the axis along which saidhigh frequency power is transmitted to eifect substantially mutualcancellation of radio frequency reflections which arise in said windowand at the edge of the aperture in said diaphragm.

References Cited in the file of this patent UNITED STATES PATENTS

